Mega Cab Speaker Replacment ???

[Bambam-x]

I am looking to upgrade my factory speakers and trying to find a little info. I know the fronts are 6x9 and rear doors are 5-1/4, question is what is the largest 6x9 spacer I can run without smashing the face of the speaker into the door panel? Looks like I only have about 2-3/4" of room without spacer for the back of the speaker, not many option in this range.

Thanks